1. Field of the Invention
The present invention relates to a pneumatic tire, more particularly to a studless or snow tire having a tread block provided with a sipe for snowy and icy roads.
2. Description of the Related Art
Typically, a pneumatic tire designed for use on icy and snowy roads has a plurality of tread blocks each of which is provided with a plurality of sipes in order to improve traction force against icy or snowy road by increasing biting edges of sipes. However, the tread block having high sipe density gives large deformation thereon when it comes into contact with the road due to low block stiffness, whereby it would be difficult to effectively get much traction force on icy road.
In order to reduce the amount of deformation of the tread block, Japanese patent application laid-open Nos. 2004-203128 and 2005-193867 disclose a sipe “A”, which is called as Miura-ori shaped sipe, shown in FIG. 4.
In a cross sectional view of the tread block being parallel to the top face S, the sipe “A” extends along its longitudinal direction in a zigzag manner. In another cross sectional view of the tread block being perpendicular to the longitudinal direction of the sipe, the sipe “A” also extends along its depth direction in a zigzag manner. The sipe “A” provides the tread block with a pair of facing sipe walls B that have a three dimensional unevenness surface comprising a plurality of inclined parallelogram planes. The sipe walls B engage each other when the tread block come into contact with the road so that the amount of deformation of the tread block mentioned above is reduced.
Since the Miura-ori shaped sipe is defined using a lot of complicated parameters such as zigzag pitches, zigzag amplitude, a combination of the mountain fold A1 and the valley fold A2, intersection angle between two parallelogram surfaces, or the like, it is very difficult to design the Miura-ori shaped sipe with three dimensional unevenness surfaces into the tread block. Accordingly, the conventional studless tires with the same tread patterns had the same Miura-ori shaped sipes for saving these design cost in spite of having different tire sizes.
However, in order to effectively minimize deformation of the tread block having the Miura-ori shaped sipe or the like, parameters for defining the shape of the sipe should be designed according to the tire sizes or the tread block sizes.
In view of above, the inventor has found an improved sipe having three dimensional unevenness surfaces that are easy to design and tightly engage each other for effectively minimizing deformation of the tread block. Such an improved sipe may be modified easily according to each tread block size or shape in a tread portion of a tire while minimizing deformation of the tread block, for example.